SO, that's where it died? Lots of big iron transformers plus a few tubes at a lot of expense for a little job? It looks like personal preferences polluted this project and turned it into (#^@&*!!!) something applicable to only a few people. Why not learn from that, and re-do it in a more efficient way that would be more widely applicable?

I had the chance today to run the "ZenEQ" concept through Tina-Ti. This is not necessarily meant to be a final design, rather a quick study how a simple circuit may perform.

This reminded me that if we require a more or less proportionate boost in dB vs. rotation we need to use potentiometers that are "S" curve, not linear. These are hard to get these days.

If using linear pots then the adjustment range gets bunched up towards the extremes. This is not necessarily a bad thing, as it makes very small adjustments very precise, the downside is that if we need a lot of correction we are left with very little usable travel.

If using switches as controls this is of course of no consequence at all, as we simply can select the correct resistances to give correct dB/vs. Attenuation.

Attached is Tina-Ti File I used (needs latest version) and some simulation results, all using 10K//1nF load. I am not sure how accurate the models buld into Tina-Ti are etc., but it would seem okayish for ballpark.

I would specify 2SK170 BL or V for the circuit, Tina-Ti lacks the model, so I made up a compound from 2N3819 & a PNP BJT, 2SK170 will show worse THD. I am for now showing ideal chokes, they can be replaced by gyrators, but I lack the time right now. For the LF choke 2H with < 120 Ohm DCR are needed, I do have a few such around, so we know they can be made.

The schematic looks like this:

The curves produced by stepping the pots in 10% intervals (this is equivalent to rotating linear pots 10% of the rotation for each step):

The frequency response with controls perfectly centred (impossible with pots) looks like this:

The LF rolloff is a result of the 2.2uF coupling capacitors shown. I habitually use this value, it works fine IME.

To illustrate two interesting EQ results, here what it looks like with all controls set halfway towards maximum cut from neutral:

This illustrates rather well how the various bands combine and with a weighting that is similar to the ISO Equal loudness peaks and through...

And here a EQ setting that can help with modern overproduced recordings, by cutting back presence and "kick" bass, and dialing the extremes up, all just a touch:

The required pot settings for this (0 = centre, 0.5 is half way between zero and the respective end, linear pots assumed) are:

Here harmonics for 1V out which adds up to 0.005% THD according to Tina-Ti:

and for 4V out which adds up to 0.05% THD according to Tina-Ti:

Clipping (1% THD) is at 5V.

Tina-Ti has a seriously funky way of showing noise, anyway, it works out ot -120dB unweighted re. 1V, which seems too good to be true. Still, might be on the money:

I hope all of this gives people some ideas for the "ZenEQ" and what sort result could be possible, to see if it is worth proceeding. To me the performance looks acceptable, we can get better measured results using Op-Amp's, but I doubt the sound will be improved.

Ciao T

Hi Thorsten!

Have you been doing any more work on the final design of the Zen-EQ? Words like ballpark tell me that this is not the final "result".

Personally I would like a tubed version of the Zen-EQ. I have been checking in and reading this thread on a regular basis..

IS this project has even been done by someone? I'm really interested. I nwould like to add control in the bass section, something like 45 hz, 80, hz, 120hz, 200hz. All the bass FREQ could be control with op amps I think with minimal sound quality loss? Can anyone tell me how to do this?